Method of forming in-situ boron nitride for ceramic matrix composite environmental protection

ABSTRACT

A method for forming in situ a boron nitride reaction product locally on a reinforcement phase of a ceramic matrix composite material includes the steps of providing a ceramic matrix composite material having a fiber reinforcement material; and forming in situ a layer of boron nitride on the fiber reinforcement material.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional application Ser. No.61/975,960, filed Apr. 7, 2014.

BACKGROUND

The present disclosure is directed to a method of forming in-situ aboron nitride layer which provides environmental protection for aceramic matrix composite.

In order to enable composite behavior, ceramic matrix composites (CMCs),such as a silicon carbide-based CMC, require a weak fiber interfacecoating which typically consists of an amorphous or turbostratic(partially crystalline) boron nitride (BN) inner layer and an outerlayer of one or more of silicon carbide (SiC), silicon nitride (Si3N4),boron carbide (B4C), or carbon (C), all of which are deposited bychemical vapor deposition. In terms of stability/reactivity, a morehighly crystalline boron nitride phase is desirable and can offergreater protection against molten silicon attack during processingcompared to amorphous or turbostratic boron nitride.

SUMMARY

In accordance with the present disclosure, there is provided a methodfor producing highly crystalline, hexagonal boron nitride via an in-situreaction between boron carbide (B4C) and silicon nitride (Si3N4).

In accordance with the present disclosure, there is provided a methodfor forming in situ a boron nitride reaction product locally on areinforcement phase of a ceramic matrix composite material, which methodbroadly comprises providing a ceramic matrix composite material having afiber reinforcement material; and forming in situ a layer of boronnitride on the fiber reinforcement material.

In another and alternative embodiment, the forming step may compriseforming the layer of boron nitride solely on a surface of the fiberreinforcement material.

In another and alternative embodiment, the forming step may compriseforming the layer of boron nitride so that the layer of boron nitridesolely surrounds the fiber reinforcement material.

In another and alternative embodiment, the providing step may compriseproviding a silicon based ceramic matrix composite material having aplurality of reinforcement fibers.

In another and alternative embodiment, the providing step may compriseproviding the silicon based ceramic matrix composite material with aplurality of fibers selected from the group consisting of siliconcarbide (SiC), Si—C—O, Si—C—N—O, Si—B—C—N, carbon, aluminum oxide,zirconium oxide, glass, mullite, and silicon nitride fibers.

In another and alternative embodiment, the forming step comprisesproviding a B4C material and a Si3N4 material and heating the B4Cmaterial and the Si3N4 material to a temperature above 1200 degreesCentigrade.

In another and alternative embodiment, the heating step may compriseheating to a temperature in the range of from 1400 degrees Centigrade to1500 degrees Centigrade to form the boron nitride reaction product.

In another and alternative embodiment, the forming step may compriseforming a hexagonal boron nitride reaction product.

In another and alternative embodiment, the B4C material and Si3N4material providing step may comprise depositing a layer of the B4Cmaterial and a layer of the Si3N4 material on the fiber reinforcementmaterial.

In another and alternative embodiment, the layer of the B4C material andthe layer of the Si3N4 material are deposited sequentially.

In another and alternative embodiment, the B4C material and the Si3N4material providing step may comprise depositing the Si3N4 material ontothe fiber reinforcement material and providing the B4C material asfiller particles in a ceramic composite material matrix surrounding thefiber reinforcement material and the B4C material being locatedimmediately adjacent the fiber reinforcement material.

In another and alternative embodiment, the B4C material and the Si3N4material providing step comprises depositing the B4C material onto thefiber reinforcement material and providing the Si3N4 material as fillerparticles in a ceramic composite material matrix surrounding the fiberreinforcement material and the Si3N4 material being located immediatelyadjacent the fiber reinforcement material.

In another and alternative embodiment, the B4C material and the Si3N4material providing step comprises applying a matrix slurry containingthe B4C material and the Si3N4 material as filler particles to the fiberreinforcement material.

Further, in accordance with the present disclosure, there is provided aceramic matrix composite material having a composite matrix and aplurality of reinforcement fibers, each of the reinforcement fibershaving a layer of hexagonal boron nitride thereon and the hexagonalboron nitride not being substantially present throughout a bulk of thecomposite matrix.

In another and alternative embodiment, the reinforcement fibers may beselected from the group consisting of silicon carbide (SiC), Si—C—O,Si—C—N—O, Si—B—C—N, carbon, aluminum oxide, zirconium oxide, glass,mullite, and silicon nitride fibers.

In another and alternative embodiment, the ceramic matrix compositematerial may be used to form a part for a gas turbine engine.

In another and alternative embodiment, the part may comprise one of acombustor liner, a turbine blade, a turbine vane, exhaust flaps andseals, and a blade outer air seal.

In another and alternative embodiment, the layer of hexagonal boronnitride may surround each of the reinforcement fibers.

Other details of the method for forming in-situ boron nitride forceramic matrix composite environmental protection are set forth in thefollowing detailed description.

DETAILED DESCRIPTION

Ceramic matrix composites, such as silicon carbide based ceramic matrixcomposites, require protective interface coatings to establish a weakinterface for composite behavior. However, the weak interface, which mayconsist of boron nitride, also offers a last line of defense againsthigh temperature oxidation. The boron nitride layer in a meltinfiltrated composite needs to be resistant to molten silicon attack.Highly crystalline boron nitride is less reactive compared to amorphousboron nitride. As used herein, the term “highly crystalline hexagonalboron nitride” refers to hexagonal boron nitride having an interplanarspacing of 3.33 Angstroms for the [002] planes. Turbostratic boronnitride would be less well defined and would have spacing of 3.5 to 3.8Angstroms for the same [002] planes. In more simple terms, highlycrystalline means more ordered. Some boron nitride coatings may bedeposited in a completely unordered structure (i.e. amorphous) or withvarying degrees of order (or crystallinity). Turbostratic BN is in themiddle—it displays regions of crystal ordering similar to that of h-BNbut does not maintain this over the entirety of the coating.

There is provided herein a method for forming in situ a highlycrystalline, hexagonal boron nitride layer using a high temperaturereaction between B4C and Si3N4. As used herein, the term “hightemperature” refers to a temperature above 1200 degrees Centigrade, suchas 1400 to 1500 degrees Centigrade. A particularly useful highcrystalline, hexagonal boron nitride layer can be formed using numerouspaths including the following: (1) a reaction between 2 depositedcoatings, such as 2 CVD coatings; (2) a reaction between depositedSi3N4, such as CVD deposited Si3N4, with B4C filler; (3) a reactionbetween deposited B4C, such as CVD deposited B4C, with a Si3N4 filler;and (4) a reaction between Si3N4 and B4C fillers via a pre-preg orinfiltration process using a pre-ceramic polymer resin or other suitablecarrier.

In accordance with the present disclosure, a ceramic matrix compositematerial having a ceramic matrix and a reinforcement phase consisting ofa plurality of fibers is provided. The ceramic matrix composite materialmay be silicon based, such as silicon carbide based. The reinforcementphase may be a plurality of fibers selected from the group consisting ofsilicon carbide (SiC), Si—C—O, Si—C—N—O, Si—B—C—N, carbon, aluminumoxide, zirconium oxide, glass, mullite, and silicon nitride fibers. Theceramic matrix composite materials may be expected to find applicationin hot-section areas of a gas turbine engine, such as the combustor andturbine. Parts which may be formed from ceramic matrix compositesdescribed herein include combustor liners, turbine blades, turbinevanes, exhaust flaps and seals, and blade outer air seals.

The boron nitride layer to be formed in situ may be formed from B4C andSi3N4. In theory, these two materials can react at room temperature.However, to form in situ the highly crystalline, hexagonal boron nitridelayer, the reactions between these two materials are performed attemperatures above 1200 degrees Centigrade, such as a temperature in the1400° C. to 1500° C. range.

The boron nitride layer which is formed, if uniform, may have athickness in the range of from 50 to 250 nm. More typically, due to thedistribution of layer thickness of typical coating processes such as CVD(chemical vapor deposition), the minimum thickness of the BN layer maybe approximately 150 nm. A useful thickness for the BN layer is in therange of from 50 to 500 nm, with a highly useful thickness in the rangeof from 50 to 300 nm.

As mentioned above, the desired BN layer may be formed in situ using areaction process. For example, the BN layer may be formed by firstapplying a layer of either B4C or Si3N4 using a CVD process, or othersuitable deposition process, on the fiber(s) forming the fiberreinforcement phase of the ceramic matrix composite and then applying alayer of the other of the B4C or Si3N4 materials also using the same orother suitable deposition process such as a CVD process. After the twolayers have been deposited, they are reacted in place by applying atemperature above 1200 degrees Centigrade to form a highly crystalline,hexagonal BN layer.

If desired, the BN layer may be formed by depositing Si3N4 on thefiber(s) forming the reinforcement phase of the ceramic matrix compositevia a deposition process such as a CVD process. The B4C may exist asfiller particles in the matrix of the ceramic matrix composite. Thematrix may be formed around the fiber(s) via a slurry or some otherpre-preg or infiltration process, whereby a fluid carrier (e.g.preceramic polymer resin) is filled with particulate material (e.g. B4Cparticles) and infiltrated into a fiber preform or otherwise formed intothe desired shape or form. The reaction would then occur at theaforementioned elevated temperatures between the two reactants, onelocated on the fiber(s) and the other located in the matrix surrounding,but immediately adjacent to the fiber(s). The resultant BN reactionproduct would be located on the surface of each fiber. At least some ofthe reaction product may be located in the matrix, surrounding thefiber, but not necessarily on the surface of the fiber.

In yet another scenario, the BN reaction product may be formed bydepositing B4C on the fiber(s) forming the reinforcement phase of theceramic matrix composite via a suitable deposition process, such as aCVD process. The Si3N4 may exist as filler particles in the matrix ofthe ceramic matrix composite. The matrix may be formed around thefiber(s) via a slurry or some other pre-preg or infiltration process,whereby a fluid carrier (e.g. preceramic polymer resin) is filled withparticulate material (e.g. Si3N4 particles) and infiltrated into a fiberpreform or otherwise formed into the desired shape or form. The reactionwould then occur at the aforementioned elevated temperatures between thetwo reactants, one located on the fiber(s) and the other located in thematrix surrounding, but immediately adjacent to the fiber(s). As before,the resultant BN reaction product would be located on the surface ofeach fiber. At least some of the reaction product may be located in thematrix, surrounding the fiber, but not necessarily on the surface of thefiber.

In yet another alternative process, the BN reaction product may beformed using a slurry or some other pre-preg process or infiltrationprocess. In these pre-preg processes, a fluid carrier (e.g. preceramicpolymer resin) is filled with particulate material (e.g. B4C and Si3N4particles) and infiltrated into a fiber preform. In the process, amatrix slurry (containing B4C and Si3N4 filler particles) may be appliedto the fibers and subsequently the impregnated fibers may be laid upinto a desired shape or form. Many different polymers may be used inthese processes, including non-preceramic polymers and polycarbosilanessuch StarPCS SMP-10 from Starfire Systems, Inc. After the matrix slurryis applied to the fibers, the impregnated fibers can be subjected to theelevated temperature for a time period in the range of from 1.0 minuteto 24 hours.

Using the methods described herein, a highly crystalline, hexagonalboron nitride layer is formed. The boron nitride layer is thus moreresistant to silicon and oxidative attack. The boron nitride layer istypically an inner layer. In some instances, the boron nitride layer maybe an intermediate, outer or only layer.

The methods described herein result in a very localized occurrence ofthe reactions, local to the reinforcement phase (primarily a fiber orfibers in a fiber composite). The reactions do not occur throughout thebulk of the composite matrix. The reaction product produced by themethods described herein is thus located on the surface of thereinforcement or just surrounding the reinforcement formed by the fiberor fibers.

Low temperature CVD BN is amorphous or turbostratic BN (a-BN or t-BN)and do not provide the protection provided by the BN reaction productlayer produced by the methods described herein.

The method described herein provides numerous benefits for protectingceramic fiber materials from molten silicon attack and environmentaloxidative attack. In-situ formation of BN via a reaction processpromotes a more crystalline boron nitride material than could be formedvia typical low temperature CVD processes. This highly crystalline boronnitride can offer additional protection against molten silicon attackduring processing. In most cases, an outer coating layer of either Si3N4or SiC is required in order to protect the BN from oxidizing duringcomposite processing. However, this outer layer (as SiC or Si3N4) doesnot necessarily offer any mechanical property benefits to the composite,but by converting the outer layer (of Si3N4) to BN, it is possible topromote “outside” de-bonding of the coating/matrix which can haveadditional mechanical property benefits.

There has been provided herein a method for forming in situ boronnitride for ceramic matrix composite environmental protection. While themethod has been described in the context of specific embodimentsthereof, other unforeseen alternatives, modifications, and variationsmay become apparent to those skilled in the art having read theforegoing description. Accordingly, it is intended to embrace thosealternatives, modifications, and variations as fall within the broadscope of the appended claims.

What is claimed is:
 1. A method for forming in situ a boron nitridereaction product locally on a reinforcement phase of a ceramic matrixcomposite material, said method comprising: providing a ceramic matrixcomposite material having a fiber reinforcement material; and forming insitu a layer of boron nitride on the fiber reinforcement material. 2.The method of claim 1, wherein said forming step comprises forming saidlayer of boron nitride solely on a surface of the fiber reinforcementmaterial.
 3. The method of claim 1, wherein said forming step comprisesforming said layer of boron nitride so that said layer of boron nitridesolely surrounds the fiber reinforcement material.
 4. The method ofclaim 1, wherein said providing step comprises providing a silicon basedceramic matrix composite material having a plurality of silicon basedreinforcement fibers.
 5. The method of claim 4, wherein said providingstep comprises providing said silicon based ceramic matrix compositematerial with a plurality of fibers selected from the group consistingof silicon carbide (SiC), Si—C—O, Si—N—C—O, Si—B—C—N, carbon, aluminumoxide, zirconium oxide, glass, mullite, and silicon nitride fibers. 6.The method of claim 1, wherein said forming step comprises providing aB4C material and a Si3N4 material and heating said B4C material and saidSi3N4 material to a temperature above 1200 degrees Centigrade.
 7. Themethod of claim 6, wherein said heating step comprises heating to atemperature in the range of from 1400 degrees Centigrade to 1500 degreesCentigrade to form said boron nitride reaction product.
 8. The method ofclaim 6, wherein said forming step comprises forming a hexagonal boronnitride reaction product.
 9. The method of claim 6, wherein said B4Cmaterial and said Si3N4 material providing step comprises depositing alayer of said B4C material and a layer of said Si3N4 material on saidfiber reinforcement material.
 10. The method of claim 9, wherein saidlayer of said B4C material and said layer of said Si3N4 material aredeposited sequentially.
 11. The method of claim 6, wherein said B4Cmaterial and said Si3N4 material providing step comprises depositingsaid Si3N4 material onto said fiber reinforcement material and providingsaid B4C material as filler particles in a ceramic composite materialmatrix surrounding said fiber reinforcement material and said B4Cmaterial being located immediately adjacent the fiber reinforcementmaterial.
 12. The method of claim 6, wherein said B4C material and saidSi3N4 material providing step comprises depositing said B4C materialonto said fiber reinforcement material and providing said Si3N4 materialas filler particles in a ceramic composite material matrix surroundingsaid fiber reinforcement material and said Si3N4 material being locatedimmediately adjacent the fiber reinforcement material.
 13. The method ofclaim 6, wherein said B4C material and said Si3N4 material providingstep comprises applying a matrix slurry containing said B4C material andsaid Si3N4 material as filler particles to the fiber reinforcementmaterial or to a preform.
 14. A ceramic matrix composite material havinga composite matrix and a plurality of reinforcement fibers, each of saidreinforcement fibers having a layer of hexagonal boron nitride thereonand said hexagonal boron nitride not being present throughout a bulk ofthe composite matrix.
 15. The ceramic matrix composite material of claim14, wherein said reinforcement fibers are selected from the groupconsisting of silicon carbide (SiC), Si—C—O, Si—C—N—O, Si—B—C—N, carbon,aluminum oxide, zirconium oxide, glass, mullite, and silicon nitridefibers.
 16. The ceramic matrix composite material of claim 14, whereinsaid ceramic matrix composite material is used to form a part for a gasturbine engine.
 17. The ceramic matrix composite material of claim 16,wherein said part comprises one of a combustor liner, a turbine blade, aturbine vane, an exhaust flap and seals, and a blade outer air seal. 18.The ceramic matrix composite material of claim 14, wherein said layer ofhexagonal boron nitride surrounds each of said reinforcement fibers.